Ceramics includes many new materials worth of attention. Among others, close attention is now paid to a piezoelectric vibrating plate (or diaphragm) formed of a highly piezoelectric ceramic having a piezo effect, which excels in the electromechanical or mechanoelelctrical trasducing action. In many cases, the known piezoelectric vibrating plate comprises a single thin metal sheet on one or both sides of which is or are laminated a piezoelectric sheet or sheets consisting of a round thin piece of 20 to 30 mm in diameter and a highly piezoelectric ceramic composed such as of zirconium, lead titanate, etc. an an electrode surface provided on the surface thereof for polarization. FIG. 12 is a sectional view showing the basic motion of a piezoelectric vibrating plate 1 of the three-sheet structure, referred to as the bimorph. When a signal voltage e is applied in between the electrode surfaces of piezoelectric sheets 2a and 2b and a metal sheet 3, expansion/contraction stresses occur at the piezoelectric sheets 2a and 2b in the opposite directions, and are, in turn, converted into shear stresses acting in between them and the metal sheet 3, thus giving rise to a vertical vibramotive force F. If the outer edge is supprorted at a fulcrum 4, then the element 1 is subjected to the convex lens-like reference vibration mode according to which its central portion vibrates in the maximum amplitude. The sound output generated by such vibramotive force F may be used for the sound generators for piezoelectric buzzers, chimes, ringers, etc. Alternatively, as shown in FIG. 13, the piezoelectric vibrating plate 1 may be built in a case 6, and be joined at its center to the apex of a sound radiator 5 for driving so as to construct a small-sized speaker, etc.
As well-known in the art, a piezoelectric ceramic has an elastic modulus substantially comparable to that of quartz crystal (E=83.times.10.sup.9 (N/m.sup.2)). The piezoelelctric vibrating plate 1 obtained by the lamination of its thin pieces onto the metal sheet 3 of the physical properties expressed in terms of reduced internal loss and high Q (sensitivity to resonance). For those reasons, it has a sharp resonance peak, and its resonance frequency f.sub.0 is generally in a high-frequency range of about 2 to 5 kHz. Since ceramic is fragile, difficulty is involved in making it thin, however, to reduce the resonance frequency F.sub.0 is practically difficult and is not economical.
Observation of the vibration phenomenon of the piezoelectric vibrating plate 1 at near the resonance point reveals, as shown in FIG. 14, the constant amplitude characteristic (d.sub.1) in the stiffness motion zone on the low-frequency side of the resonance peak f.sub.01, and the constant velocity characteristic (V.sub.1) in the inertial motion zone on the high-frequency side. Now, let's presume the motion of a small-sized speaker, shown in FIG. 13, from an equivalent circuit diagram, shown in FIG. 15. Then the mechanical impedances z.sub.1 and z.sub.0 of the piezoelelctric vibrating plate 1 and the cone sound radiator 5 form together a series-connected circuit. In addition, z.sub.1 is much higher than z.sub.0. For those reasons, a velocity V.sub.0 flowing in the cone sound radiator 5 is entirely governed by z.sub.1, so that the movement of the radiator 5 is made similar to that shown in FIG. 14.
According to the acoustic theory, when it is desired to allow the acoustic radiator to radiate a constant sound pressure within a certain band in a free space, it is in principle required that the sound radiator vibrate at a constant velocity. Hence, referring to the radiating sound pressure characteristics of the conventional small-sized speaker of FIG. 13, a relatively high sound pressure is attained on the high-frequency side of the resonance point f.sub.0, but, on the low-frequency side, the output sound pressure drops sharply with the frequency. As mentioned in the foregoing, since the resonance point f.sub.0 of the piezoelectric vibtating plate 1 is found at about 2 to 5 kHz, the tone of reproduced sound becomes poor. This is because the high-frequency portion only is stressed, and the low-frequency portion is defficient. In addition, since the piezoelectric sheets 2a and 2b are of high Q, the resonance point f.sub.0 is associated with a sharp resonance peak, and irregular responses occur with the frequent occurrence of high-harmonic strains, and the output sound pressure level drops in the middle- and low-frequency ranges. The resulting speaker is of no general use. In order to obviate such drawbacks, it has so far been proposed to, on the one hand, reduce f.sub.0 with the use of a special large-sized piezoelectric vibrating plate, and on the other hand, apply a viscoelastic resin on the surface of the piezoelelctric sheets 2a and 2b or the vicinity of the fulcrum 4, whereby lowering Q. However, this is only an inefficient means, and is expected to be less effective. This is because z.sub.1 is too high, and the resonance point f.sub.01 is found near the upper limit of the audible range (3 to 5 kHz). To control freely this is not substantially possible at all by any conventional means.